This invention relates to automatic repairing methods for a broken wire electrode of wire electrode electric discharge machining devices.
It is well known that the wire electrode electric discharge machining devices are capable of machining works of electrically conductive materials easily and precisely into complicated forms irrespective of the hardness of the works. The wire electrode, however, of the wire electrode electric discharge machining devices is thin, usually from about 0.03 mm to 0.3 mm in diameter, and is consumed by the discharge during the electrical discharge machining. Thus, the wire electrode may be broken by the tension applied thereto during the electrical discharge machining, and the machining may thus be interrupted. Thus, Japanese Patent Publication (Kokoku) No. 63-28731, for example, proposes an automatic repairing method of the wire electrode of the wire electrode electric discharge machining devices.
FIG. 10 shows a cross-section of a wire electrode electric discharge machining device by which the wire electrode is repaired according to the conventional method, and FIG. 11 is a schematic perspective view showing the trajectory of the wire electrode during the repairing of the wire electrode in accordance with the conventional repairing method. When the wire electrode 3 is broken, the numerical control device 8 drives the X- and Y-axis driving devices (not shown) to translate the work 1 via a XY machining table (not shown), such that the wire electrode 3 is moved from the wire breaking position 11 to the machining start position 12. When the wire electrode 3 is at the machining start position 12, a new length of the wire electrode 3 is fed from the feeding-side wire guide unit 4 to the receiving-side wire guide unit 5, while the water jet 7 is ejected from the port of the feeding-side wire guide unit 4. The wire electrode 3 fed from the wire electrode feeding mechanism of the feeding-side wire guide unit 4 is constrained by the water jet 7 to pass through the machining start hole 2 and the receiving-side wire guide to be reeled on a wire receiving mechanism or to be received into a container. The wire electrode 3 is thus spanned ready for the electrical discharge machining.
Thereafter, the wire electrode 3 is moved relative to the work 1 along the machining trajectory 13 within the machined groove 14 and is returned to the wire breaking position 11. Thereafter, the machining power source 15 is turned on, to generate the discharge energy and restart the electrical discharge machining of the work 1. This automatic repairing operation of the broken wire electrode may be repeated an indefinitely large number of times if the numerical control device 8 is so set. Generally, however, a predetermined upper limit is set such that the automatic repairing of the broken wire electrode is repeated up to that predetermined upper limit.
The above wire electrode electric discharge machining device thus is capable of automatic operation. However, the conventional repairing method still has a disadvantage. Namely, it often happens that the number of repairing operations of the wire electrode reaches the preset upper limit and intervention of a human operator becomes necessary.
The wire electrode 3 moves in the narrow machined groove 14 whose width is only about 0.02 mm to 0.08 mm greater than the diameter of the wire electrode 3. The width g of the machined groove 14, however, may become smaller than the diameter d of the wire electrode 3 along the machining trajectory 13 as indicated in FIG. 11, due to the deformation of the work 1 occasioned by the release of the interior stress, or due to the deviation, peculiar to the wire electrical discharge machining, of the machined groove from the target contour at the corners of the machined trajectory. When this is the case, the re-spanned wire electrode 3 cannot pass the narrow gap g of the machined groove 14, and may again be broken before reaching the wire breaking position 11. Under such circumstances, the broken wire is detected immediately at the wire breaking position 11 at the restart of the machining. The wire repairing operations are thus repeated only to be broken again at the narrow gap g until the number of repairing operations reaches the preset upper limit.
When the number of repairing operations reaches the predetermined limit, the repairing operations are no longer repeated. Thus, the machining is terminated, leaving a portion of the target trajectory unmachined. Alternatively, when the wire electrode electric discharge machining device is operated according to a program for machining a plurality of forms (trajectories) in the work 1 and the next form is set, the machining program proceeds to the machining of the next form, leaving a portion of the interrupted trajectory unmachined.
When the automatic machining is terminated leaving a portion of the target trajectory unmachined, a human operator must pass the wire electrode 3 through the machined groove 14 and the receiving-side wire guide unit 5 at the wire breaking position 11, and, after spanning the wire electrode 3, restart the electrical discharge machining by supplying the working liquid, moving the wire electrode 3, and turning on the machining power source 15.
In the case where a program for machining a plurality of forms are terminated leaving a portion of the interrupted trajectory unmachined, the operator must drive the device such that the wire electrode 3 is moved to the interrupted point without machining the work 1. When the wire electrode reaches the wire breaking position 11, or is moved to a point just before the wire breaking position 11 along the machining trajectory, the operator stops the device, and then restarts the electrical discharge machining after feeding and spanning the wire electrode and performing the necessary restarting operations, as described above. This operation is time-consuming and burdensome for the operator.
Since the wire electrode 3 cannot be moved along the machining trajectory 13 within the machined groove 14 and the automatic repairing operations fail as many times as they are tried, intervention of a human operator becomes necessary. Thus, according to the conventional repairing method, automatic machining for a prolonged length of time cannot be accomplished smoothly without operator's intervention.
Further, when the work 1 with an unmachined portion is left for a long time, rust develops in the machined groove 14, such that the wire electrode 3 tends to be broken more often than before after the restart of the machining by the operator. Then, due to repeated restarting operations, the width of the discharge gap exceeds the target value thereof by far, and the work 1 becomes defective.
Further disadvantage of the conventional wire repairing method is that it is time consuming even when the device is operated automatically without operator's intervention. Namely, when the wire electrode 3 is moved along the machined groove 14, the positional deviation (generally known as the droop of the wire electrode) becomes the greater as the velocity of the wire electrode 3 increases. Thus, when the wire electrode 3 is translated at a high velocity, the actual trajectory of the wire electrode 3 is greatly deviated from the target trajectory, such that the movement of the wire electrode 3 is interrupted by the surface of the machined groove 14 at the corners thereof and the wire electrode 3 is broken. Thus, the velocity of the movement of the wire electrode 3 is generally limited under 100 mm per minute. When the length of the machining trajectory is as long as several thousand millimeters, each repairing operation requires several tens of minutes. In such case, the conventional wire repairing method is thus extremely inefficient.